serf: Fix bug where knights are not added to defenders list when entering castle
serf: Avoid inifinite loop in LOST state when player does not have any land left
game: In demolish_building() disconnect flag before update land ownership

This avoids a crash when updating land ownership tries to demolish the flag.
game: Properly dismiss the serf serving the castle, when the castle is demolished

This fixes #117.
Merge fixes from release-0.1 related to demolishing castle after winning a battle there